How to duplicate a camera?
it is definitely possible.
You even did it already ;) Just: the new camera has got exactly the same settings as the original. Means: In the 3D view both lay exactly over each other and appear as one.

Press "G" for grabbing and move mouse for moving the new (selected) camera after duplicating. Once you reached the wanted position, press the left mouse button.

Alternatively you can use the short cut "Shift + D" for duplicating and grabbing the camera. Now you're able to reposition the camera after duplicating it.

Greets
Jaspa